The present invention relates generally to magnetic resonance (MR) systems and, more particularly, to an apparatus to limit coupling between moveable coils of an RF coil assembly of an MR system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
It is generally well-known that RF coil loops of an MR system placed adjacent to each other couple heavily by mutual inductance. This coupling causes detuning of the coil loops, loading of the loop, and degradation of image quality. To eliminate the coupling, a number of techniques and coil designs have been developed. For example, the loops may be critically overlapped to cancel the coupling, inductively coupled to cancel the coupling, capacitively coupled to cancel the coupling, or reduced by high impedance resonant circuits that turn off the loops. It has been found that the first three designs are sufficient for only one orientation of loop proximity. The fourth design has been found not to provide sufficient isolation to cancel loop coupling.
Moreover, if the coil loops are flexible, or can move relative to each other, the isolation designs described above often fail. That is, as the orientation of loop proximity changes, the RF coil loops must either be retuned to minimize the coupling at the new orientation or a different RF coil loop assembly must be used that is tuned to the new orientation. MRI system operators must therefore sacrifice patient throughput by devoting time to returning of the coil loops. Additionally, selecting a different RF coil loop assembly already tuned to the new orientation not only requires time and effort away from image acquisition, but also requires that an imaging facility maintain an inventory of a number of RF coil loop assemblies to satisfy the many orientations that may be used to acquire diagnostic data. Maintaining a large inventory of RF coil loop assemblies is cost prohibitive, and, despite extensive cost, may not be exhaustive of the coil assemblies needed.
It would therefore be desirable to design a system capable of providing an RF coil assembly such that loop isolation is maintained over a wide range of loop positions or orientations.